A sandwich-type enzyme-linked immunosorbent assay (ELISA) for the quantification of human apolipoprotein A-I (apoA-1) was developed using monoclonal antibodies. For this assay. we used three monoclonal antibodies to trap and detect apo A-1. HDAl16 and HDAI5 monoclonal antibodies were used for trapping apo A-I and HDAI8 monoclonal antibody was for detecting apoA-I. These three monoclonal antibodies were produced by immunizing mice with high density lipoprotein (HDL) isolated from human plasma. By immunoblot analysis. these three monoclonal antibodies were specific to apoA-1 and showed no cross-reactivities with other plasma proteins. The results of competition assays for epitope cross-reactivity test also verified that these monoclonal antibodies identified separate and distinct epitopes on HDL and apoA-1. Affinity constants of monoclonal antibodies were measured by ELISA. Their association constants ranged from 10^7 to 10^8 M^(-1). For this assay, pure apoA-I was isolated by affinity chromatography using monoclonal antibodies. In this sandwich assay. the amount of HRP-labeled HDAIB bound to apoA-I trapped by HDAl16 and HDAI5 was proportional to apoA-1 concentration in the range of 0 to 500ng/ml. ApoA-1 concentration in plasma was calculated from the linear regression equation of standard curve. The precision and reliability of the assays are reflected in the low intra- and interassay coefficients of variation that averaged 3.25% and 4.30%, respectively. This assay is sensitive, simple. reproducible. convenient in incubation interval, and does not use radioisotope: thus it can be widely applied in clinical laboratories.